In recent years, in all technical fields, energy saving, resource saving and environmental measures have become important subjects, and various technical developments have been made. As one of them, a photo-curable resin composition has been developed and used in applications to e.g. coating materials, adhesives, coating agents, printing ink vehicles, resist inks, relief printing plate materials, optical fiber coating materials and optical shaping materials. Such a photo-curable resin composition comprises a photo-curable oligomer, a photo-polymerizable monomer, a photo-polymerization initiator, a sensitizer, a colorant and other additives and contains substantially no volatile solvent, and it undergoes a curing reaction by irradiation with light rays having a certain specific wavelength to give a cured product excellent in flexibility and adhesive properties.
The photo-curable oligomer to be used here, is usually one having from one to several (meth)acryloyl groups as photo-curable functional groups in its molecular structure. Among such oligomers, a urethane (meth)acrylate oligomer obtainable by reacting a polyol and a polyisocyanate compound, is used in a wide range of fields, as its cured product exhibits excellent performance in the toughness, hardness, chemical resistance, flexibility, adhesive property, light resistance, low temperature characteristics, etc.
With respect to such a urethane (meth)acrylate oligomer, molecular designing can easily be done by changing the starting material variously depending upon the particular purpose of its use. Especially, it is easy to change the performance by the polyol to be used. When a low molecular weight polyol having a molecular weight of about a few hundreds, is used, a hard and brittle cured product will be obtained, and when a high molecular weight polyol having a molecular weight of from a thousand to a few thousands, is employed, a flexible and tough cured product will be obtained. As such a high molecular weight polyol, a polyoxyalkylene polyol, a polyester polyol, a polycaprolactone polyol or a polycarbonate polyol is, for example, employed.
However, if a high molecular weight polyol is used, the viscosity of the resulting urethane (meth)acrylate oligomer will be high, and it has been difficult to let the urethane moiety have a high molecular weight so as to obtain sufficient flexibility. Further, because of the high viscosity, there has been a problem in working efficiency, and as shown in JP-A-9-48830, it used to be necessary to employ a low molecular weight (meth)acrylate compound as a viscosity-reducing agent.
As a low viscosity polyol, a polyoxyalkylene polyol is known. However, if a common polyoxyalkylene polyol as shown in JP-A-10-95640 is used, since such a polyol contains a substantial amount of a by-product monool, the curing tends to be slow, and there has been a problem that no adequate physical properties can be obtained, or the surface tackiness of the cured product remains.
Under these circumstances, the present inventors have conducted an extensive study, and as a result, have found it possible to obtain a photo-curable composition which has a low viscosity and excellent curability and which gives a cured product having little surface tackiness, by employing a certain specific urethane (meth)acrylate oligomer based on a polyoxyalkylene polyol having a low total degree of unsaturation (i.e. a small monool content). The present invention has been accomplished on the basis of this discovery.